This invention relates to apparatus for supporting a rotating member or shaft which may become subject to high unbalance forces under operating conditions.
Rotating shafts, particularly those associated with modern day gas turbine engines, may encounter abnormally severe unbalance conditions during the course of engine operation. Typically, these conditions may arise when elements, such as one or more of the plurality of blades mounted on a disc attached to the shaft, are damaged and separate from the rotating disc. The combination of severe unbalance and critical speed within the operating range of the rotating shaft may effect significant vibratory loads that could result in severe rubs or, in the extreme, rapid bearing or structural failure.
Prior art attempts have been made to address the severe unbalance conditions caused by separation of a rotating blade. Typically, attempts have been made to place the critical speed outside of the operating range of the engine by adjusting the mass and flexibility of the rotating element. Since this approach increases the weight of the engine, fuel consumption of the engine has correspondingly increased. Additionally, since any number of rotating blades may separate from the engine, the exact vibration response, and hence critical speed, is difficult to predict. For these and other reasons then, adjustments to the mass and critical speed of the engine have not proved to be entirely satisfactory.
Another prior art approach, addressing the severe unbalance condition described above, provided a damper in the bearing support structure which dampened the radial movement of the shaft. More specifically, dampers employing a squeeze film concept were found to be particularly adaptable in reducing the consequences of a severe unbalance condition. However, this approach introduces additional operating clearances, associated with the damper, into the bearing support. These clearances may be effective, during certain conditions under normal operation of the engine, to permit radial movement of the rotating shaft. In other words, the shaft has a soft, rather than firm support under normal engine operating conditions. The rotating blades associated with the shaft then rub on the stationary components of the engine causing larger clearances between the blade tips and the stationary components to exist under normal operating conditions. Hence, the efficiency of the engine is reduced. The present invention addresses the problem associated with soft support of the rotating shaft.